fs: allow setting a write buffer for multithread

when multi-thread downloading is enabled, rclone used
to send a write to disk after every read, resulting in a lot
of small writes to different locations of the file.

depending on the underlying filesystem or device, it can be more
efficient to send bigger writes.

docs/content/docs.md

@@ -1511,6 +1511,25 @@ if you are reading and writing to an OS X filing system this will be
 
 This command line flag allows you to override that computed default.
 
+### --multi-thread-write-buffer-size=SIZE ###
+
+When downloading with multiple threads, rclone will buffer SIZE bytes in
+memory before writing to disk for each thread.
+
+This can improve performance if the underlying filesystem does not deal
+well with a lot of small writes in different positions of the file, so
+if you see downloads being limited by disk write speed, you might want
+to experiment with different values. Specially for magnetic drives and
+remote file systems a higher value can be useful.
+
+Nevertheless, the default of `128k` should be fine for almost all use
+cases, so before changing it ensure that network is not really your
+bottleneck.
+
+As a final hint, size is not the only factor: block size (or similar
+concept) can have an impact. In one case, we observed that exact
+multiples of 16k performed much better than other values.
+
 ### --multi-thread-cutoff=SIZE ###
 
 When downloading files to the local backend above this size, rclone